The present invention relates to an airbag cover which closes an exit opening of an airbag unit in a motor vehicle.
Usually, such covers are visually and haptically adapted to their installation surrounding. Insofar as a driver airbag is concerned, the cover has to be matched to the surface structure and colour of the steering wheel sheating. Front passenger airbags are arranged below the instrument panel so that the surface of the cover has to correspond with the surface of the instrument panel as far as possible. The situation is similar for side airbags and all other airbags to be installed in motor vehicles.
It can easily be understood that adapting the cover to its installation surrounding often poses great problems, because the latter possibly consists of materials which are not or only partly suitable for manufacturing a cover. Typical covers are therefore configured normally with two or more layers, the internal layer being provided with the required structural strength, whereas the outer layer is taylored so as to adapt to the installation surrounding. In most cases, the cover is a part of a separate airbag module which is designed for the installation on a steering wheel, an instrument panel or other internal parts of a car body. In these cases, the cover is often designated as xe2x80x9ccover capxe2x80x9d, the circumferential edge of which is adapted to the size of the installation opening and the tear area of which, in contrast, is smaller.
An airbag cover may constitute an integral component of an instrument panel or of another internal part for motor vehicle bodies, where the adaptation to the installation surrounding generally poses no problems, but wherein the exact positioning of the tear line and the dimensioning of the predetermined breaking lines with respect to their strength can become difficult.
Hitherto it is assumed that, in the case of airbag covers configured in two or more layers, the tear lines intended as predetermined breaking lines have to be arranged in all layers such that they have the same length and coincide with each other, so that a defined resistance to tearing can be reproducibly realized within narrow limits, because in this respect one cannot accept an incalculable hindrance on the sudden deployment of the airbag. This, however, can be achieved in the case of covers configured in two or more layers only with significant production expenses, because for producing the individual layers separate operating steps are required and, mostly, also separate form tools.
The present invention provides an airbag cover which can be produced at a far more favorable price, without posing the need of accepting incalculable resistances on the sudden inflation of the airbag along predetermined lines.
According to the invention, an airbag cover is provided which consists of two or more layers, a full-length tear line intended as a predetermined breaking line being provided only in a first one layer of these layers.
Tests have surprisingly brought the result that there is no need for a full-length tear line in the second layer and that the second layer in fact tears open independently of the tear line predefined in the first layer when the airbag is inflated. The dynamic forces during the tearing process obviously are of more importance than the static tensile strength so that any resistances to tearing hardly play a role when the first layer is torn open under the influence of the gas pressure wave acting in the airbag. Therefore, it is sufficient to configure a tear line intended as a predetermined breaking line in the second layer, which tear line is shorter than that of the first layer. Furthermore, it has been found that it is sufficient to configure the tear line of the second layer so as to be at least partially coincident with the tear line of the first layer.